Various techniques are known for reducing the measurement time needed for MR imaging. For instance, techniques are known for exciting the core magnetization simultaneously in a plurality of slices of the region under examination and acquiring MR imaging data simultaneously from the plurality of slices. Such techniques may be referred to as simultaneous multislice (SMS) imaging.
There are a large number of different methods for performing SMS imaging. A parallel imaging technique called partial parallel acquisition (PPA) is conventionally used to separate the MR imaging data and includes a slice-specific reconstruction dataset for each of the slices. A recently introduced method is the method introduced by Setsompop et al. (MRM 2012), “Blip-controlled aliasing in parallel imaging” (Blipped-CAIPI), which is described in more detail in the article by Setsompop, Kawin, et al., “Blipped-controlled aliasing in parallel imaging for simultaneous multislice echo planar imaging with reduced g-factor penalty.”, Magnetic Resonance in Medicine 67 (2012), 1210-1224. This method uses what is known as a multiband pulse to excite a plurality of slices simultaneously. In addition, the pulse waveforms for all the bands are summed, resulting in a multiband pulse modulated into a carrier band. For each excited slice, a linear phase ramp is added in k-space along the slice direction.
In order to reduce losses relating to the g-factor, offsets between the slices are produced during readout either by gradient blips on the slice axis or by modeling the phase of the RF pulses. After acquisition, the simultaneously excited slices are combined into a single slice. The slices may be separated from one another in post-processing using a slice-GRAPPA method (Setsompop et al., MRM 2012). If an acceleration is additionally applied in the slice plane, reconstruction in the slice plane is performed in a second act using the GRAPPA method.
The turbo-spin-echo (TSE) sequence is a sequence that is widely used in the clinical field for examining numerous body regions. The TSE sequence includes a plurality of echo sequences, with a plurality of phase encoding lines of the full k-space being acquired in each echo train after one excitation pulse. This is achieved by using refocusing pulses to refocus the spins after each readout line is acquired. Thus, compared with the conventional spin-echo (SE) sequence, the acquisition time is reduced by the number of refocused echoes in an echo sequence (what is known as the turbo factor).
To facilitate the separation of the combined multiband data, a reference scan is acquired in addition to the multiband data using a single band, which covers all the slices. Current SMS-TSE implementations contain a TSE or gradient-echo (GRE) reference scan before the acquisition of the SMS data. This reference scan is then used to perform both the calibration of the kernels for the slice-GRAPPA method and the calibration of the kernels for the GRAPPA method in the slice plane. After the slice-GRAPPA reconstruction is carried out for generating slice data of the slices, (which slice data is subsampled in k-space), calibration data for calculating the kernel of the slice-GRAPPA method is subsampled likewise. Reference lines are consequently deleted in the current implementation. For example, a reference scan for a SMS 2, iPAT 2 (R=2) acquisition includes 64 k-space lines. For the GRAPPA method in the slice plane, all the 64 reference lines may be used. For SMS, only 64/R=32 reference lines may be used. For iPAT 3 (R=3), only 64/3=21 reference lines may be used. This may lead to a reduced signal-to-noise ratio, separation artifacts and incorrect assignments of MR signals to the individual slices. The number of scanned reference lines may be increased to offset these disadvantages. This is inefficient, however, because the proportion of deleted data remains the same and yet the scan time is still extended. Besides the longer scan time, other disadvantages also arise, such as a greater probability that the patient moves during the reference scan, an increased SAR load, and, in the case of a TSE reference scan, T2 decay and a reduced signal as a result.
U.S. Patent Application Publication No. 2016/0313433 A1 and U.S. Patent Application Publication No. 2015/0115958 A1 disclose simultaneous multislice measurement methods.